Composition for sintered barium cathodes



United Sttes atent Patented Apr. 14, 1959 ice COMPOSITION non SINTERED BARTUM= CATHODES Huber, Jean Frey and Jean Remand) Paris;-

Fra'nc'e, assignors to Compagnie Generale de' Telegraphite Sans Fil, a corporationof France N Drawing. Application June 13, 1955 Serial No. 515,171

Claims priority, application France June 16, 1 954 13 Claims. (Cl. Z9 182 The invention relates to tablets constituting the er'nis sive'f-part' of sintered barium cathodes. These tablets are gener Iy' obtained by agglomer'ating, for instance by compressing and sintering a mass of powdered materialsccntaining the cation Baand variousmetals.

lt is an object of the invention to provide'a" composition for sintered barium cathodes of the type comprising: a 'barium composition, a reducerand a carrier.

In thecomposition of the invention, the bariuni coinposition has the general formula nX|-M;Oy, X designating either BaO alone or a combination ofBaOwith CaOor S'rO, or CaO+SrO, M designating a metal hav in'ga valence at'least equal to 4; n, x, and y being the atomic or molecular proportions of the'constituents. This barium-composition willbe hereinafter calledstabilized bariumoxide. The above general formula mayi ndicate" either a solid solution, a chemical compound, or anyother' combination of the oxides. In the preferred composition of the present invention, the barium composition has the general formula 3B aO+WO i.e., basic barium" tungstate. This barium composition will becalledhereinafter stabilized barium oxide.

The reducer of the composition according to themvehtion' is a metal capable of reducing said bariumco'mpositionwhen-the two bodies are in the solid state, and

the carrier is a refractory metal. The same rrietal nia'y alsoibe used as reducer and as carrier.

According to a preferred-embodiment of the invention, the molecular proportion of the carrier is 50 to 97% of the composition.

According to another embodiment of the invention, said barium composition further includes strontium oxide. According to still another embodiment of the invention,

saidbarium composition further includes strontium oxide and calcium oxide.

Anoflier object of the invention'is to provide 'a process ofinianufacture of tablets for sintered barium cathodes. This process comprises the following steps: intimately mixinga carbonate containing at least the cation Ba and a stabilizingrnetal oxide, at a temperature between'430 C. and 1,230 C.; powdering'the barium composition thus obtained and designated stabilized barium oxide, mixing the stabilized barium oxide with a reducer and a carriei, both of which having been finely powdered; subjecting this mixture to a high pressure and sintering the'same at high temperature. The surface of the sintered tablet is then preferably ground.

According to a preferred mode of carrying out the invention, said barium composition is basic barium tungstate (3BaO+WO the reducer is aluminum, the sintering is effected in a neutral atmosphere at a temperature between 1315 C. and 1800 C., and its duration isbe tween 3hours and 5 minutes;

The invention Will be more clearly understoodfrom' the following examples to which the scope of the invention is not intended to be limited.

2 Example 1 There is first obtained an'intimate mixture of barium carbonate BaCOgand"tungstenoxide'WO which is powdered for several hours in the proportions of 3 molecules of BaCO to l moleculeof W03; This mixture is heated ma crucible in a'furnace having a neutral' atmosphere,

such as argon, for half an hour to one hour at a"tem-' perature between 430 C. and 1230' C. After baking, the mixture is powdered, for instance, by a ball mill, for a period ranging f'rom several hours" to 30 hours.

According to the duration ofgr'indin'g, the resultant" size of the grains varies from 30 to lrnicron. Double oxide (3BaO+WO obtained by means; of the aforemen tioned operations, is mixed and powdered with alurninuni and'tungsten for several hours, in th "pioporticns of- 5' molecules of Ba WO to 15 atoms Al and atoms of W, the size of the grains of alur'ninum and tungsten being between 1 and 50 microns. A certain amount of this mixture is compressed by means of a press capable of providing a pressurebetween l and" 30*metric tons/crnF.

The tablet obtained is 'positionedon itssupport orin a= container permitting its attachment to the support, and

is sintered in an argon atmosphere at a tempe'rature betwe'en--l3l5 C. and l800= C. for a period ranging from The surface of the sintered tablet:

the-'emissive zone.

(5) High thermal efliciency;

(6) Solidity and strength when'subject'ed tomechanical stresses and heat'changes;

(7) Long life'without metallization'of the electrodes disposed in front of the cathode.

(8) Minimum-emission of I gasduring degassing? and" activation.

(9) Ease of machining.

(10) Easier adjustment of'the liberation rate ofbariunf andof the formationby diifusionofthe barium filmto suit operation of conditioner of the cathode;

Cathodes utilizing thistablet are capable ofop'erating." between830 C. and l,250 C., andofsupplying severalamperes per sq. cm. underconditions-of continuousop era'tion and several tensof amperesper sq. cm. under conditions of pulse operation.

Some of the above-indicated advantages may beat:

tributed to the formation of a'filrn of activeb'arium ad sorbed at the surface of thecarrier (tungsten).

sten. As this film is continuously subjected to destructive efiects in the course of the operation ofth'e'cathode it must be continuously renewed. This renewal is ob tained by reason of the fact that the aluminumreducr liberates barium from the supplier of barium; i.e. (3BaO-l-WO said barium diffuses towards: the Slitface of the carrier at the high temperature of operation of the cathode. The rate of liberation of the barium atoms depends, in particular, on" the" proportions ando'n" the nature of the supplier; It'has' b'eenfoun'd'that, with thecomp'osition of bariuni'u'sed'according to the invention; the rate of diffusion may: be correctly adjusted to obtain thedesired densityof emitted current.

The function of the stabilizing oxide W0 is to stabilize chemically the oxide BaO, which should be deterior- This film of bariumdecreasesthe work function of'the tung' 3 ated in the presence of atmospheric air before sintering of the tablet.

Example 2 The same process of manufacture as in Example 1 is used with the following composition:

(3BaO+WO +15Al+80Mo Example 3 The same process of manufacture as in the foregoing examples is used with the following composition:

The tungsten performs the function of reducer, carrier and stabilizing oxide cation.

As all the bodies included in the formula mentioned above are very refractory, there is no danger of formation of vapors which are harmful to the cathode or to the other electrodes of the tube in which the cathode is placed.

Example 4 The composition used is:

20(3BaO+WO +W+70Mo the reducer being tungsten and the carrier being molybdenum.

It must be understood that the scope of the invention is not intended to be limited to the specific examples given above. I

It is contemplated within the scope of this invention that the cathode composition may be made by mixing with a refractory metal carrier and a metal capable of reducing barium tungstate a powder, such as or 3BaO+WO -(SrO) WO -(CaO) WO and then compressing and sintering the mixture.

In a general way, an oxide of one of the following metals may be used as a stabilizer: W, Th, Ta, Nb, Mo, Ti, Zr. More generally it has been found that there may be used any metal having a valence at least equal to 4.

Stabilized barium'oxide may be obtained either from barium carbonate, as in the first example described above, or from double carbonates such as BaCO -SrCO or triple carbonates such as BaCO -SrCO -CaCO In certain cases, the presence of another carbonate, in addition to barium carbonate, facilitates the stabilization of BaO or the formation of solid. solutions permitting the stabilized compound to be enriched with BaO.

Similarly, the reducer may be selected from the group consisting of metals such as Ti, Zr, Th, Ta, Nb, W, Mo, Al, Si, which are capable of reducing BaO at high temperatures, by a reaction in the solid state, BaO and this reducer being in the form of sintered powders.

The carrier is a refractory metal such as W, Mo, Re, Nb.

It has been seen in some of the foregoing examples that a single metal may perform several of the respective functions of stabilizer, reducer and carrier. It is obvious that it must then possess all the properties required for the various functions it performs simultaneously. For example, W in the Example 3 is capable of reducing BaO in the solid state and is sufficiently refractory for performing the function of carrier.

What is claimed is:

1. Composition for a tablet used as emissive portion of a sintered barium cathode consisting essentially of Percent 3BaO+WO (basic barium tungstate) 5 Al W 80 2. Composition for a tablet used as emissive portion of a sintered barium cathode consisting essentially of Percent 3BaO+WO (basic barium tungstate) 5 Al 15 Mo 3. Composition for a tablet used as emissive portion of a sintered barium cathode, consisting essentially of Percent 3BaO+WO (basic barium tungstate) 20 W 80 4. An emissive cathode composition for a tablet used as emissive portion of a sintered barium cathode, consisting essentially of between 50 to 97 percent of refractory metal carrier, and of a balance comprising at least a chemically bound barium composition having general formula nBaO+M O wherein M is a metal having a valence at least equal to 4, and n, x, y are integers indicating molecular or atomic proportions of the constituents of such chemically bound compositions satisfying said general formula, said cathode composition containing a proportion of a metal reducer capable of reducing the barium composition at the temperature of operation of said cathode.

5. An emissive cathode composition as claimed in claim 4, wherein M is a metal selected from the group consisting of W, Th, Ta, Nb, Mo, Ti, Zr.

6. An emissive cathode composition as claimed in claim 4, wherein said balance comprises said proportion of reducer of said barium composition, said reducer being selected from the group consisting of Ti, Zr, Th,.

Ta, Nb, W, Mo, Al, Si, the selection of carrier and reducer being so made that the same element is not used as said reducer and said refractory carrier.

7. An emissive cathode composition as claimed in claim 4, wherein said refractory carrier metal is selected from the group consisting of W, Mo, Re, Nb.

8. An emissive cathode composition as claimed in claim 4, wherein said barium composition is basic barium tungstate 3BaO+WO 9. An emissive cathode composition as claimed in least a chemically bound barium composition having general formula wX+M O wherein X includes barium oxide and strontium oxide, M is a metal having a valence at least equal to 4, and n, x, y are integers indicating molecular or atomic proportions of the constituents of such chemically bound compositions satisfying said general formula, said cathode composition containing a proportion of a metal reducer capable of reducing the barium composition at the temperature of operation of said cathode.

13. An emissive cathode composition according to claim 12, wherein X further includes calcium oxide.

References Cited in the file of this patent UNITED STATES PATENTS 1,363,162 Myers et al Dec. 21, 1920 2,488,731 Lambert Nov. 22, 1949 2,700,118 Hughes et al. Jan. 18, 1955 

1. COMPOSITION FOR A TABLE USE AS EMISSIVE PORTION OF A SINTERED BARIUM CATHODE CONSISTING ESSENTIALLY OF PERCENT 3BAO+WO3 (BASIC BARIUM TUNGSTATE) 5 AL 15 W 80 